Phenol is manufactured via air oxidation of cumene to cumene hydroperoxide (CHP), followed by acid-catalyzed cleavage of CHP to phenol and acetone. CHP decomposition is a very exothermic reaction which is normally carried out on a commercial scale in continuous stirred or back-mixed reactors. In such reactors only a small fraction of CHP remains at any given time and the reaction medium consists essentially of the products of decomposition of CHP, i.e., phenol and acetone, plus any solvent (e.g., cumene) and other materials added with CHP to the reactor. During cumene oxidation small amounts of dimethyl phenyl carbinol (DMPC) and acetophenone are also formed. In the presence of acid catalyst, DMPC dehydrates to AMS, a useful by-product. Very high yields of AMS can be obtained from pure DMPC, e.g., 98 percent yield upon dehydration over acidic silica at 300° C. In the presence of phenol, however, and more specifically in phenol/acetone/cumene which is solvent in decomposition of technical CHP/DMPC mixtures, the AMS yield is normally about 50–60 mol percent of the DMPC. Main by-products are AMS dimers and cumylphenol which have no commercial value. Formation of cumylphenol also reduces the phenol yield.
G. G. Joris, U.S. Pat. No. 2,757,209, teaches that the amount of AMS dimers and cumylphenol formed can be substantially reduced by carrying out the reaction in two stages. In the first stage CHP is decomposed in a stirred or back-mixed reactor in the presence of small amounts of sulfur dioxide as catalyst and water as catalyst moderator. Preferred conditions are: temperature 45–65° C. sulfur dioxide 50–500 ppm, water 2–5 weight percent. Under these conditions the CHP concentration in the reaction mixture withdrawn from the reactor is less than 5 percent but more than 1 percent by weight. In the second stage, the mixture withdrawn from the first reactor is heated in a second reactor, optionally with additional catalyst, in order to decompose residual CHP and to effect the dehydration of DMPC to AMS. This second reactor is either a batch reactor, or a continuous plug-flow reactor. Preferred conditions are: temperature 110–120° C., reaction time 5–15 minutes. Care must be taken to stop the high temperature reaction once AMS formation is completed so as to minimize dimerization of AMS or the reaction of AMS with phenol to form by-products.
U.S. Pat. No. 4,358,618, to Sifniades et al. teaches that the amount of AMS dimers and cumylphenol formed is minimized by carrying out CHP decomposition in three stages. In the first stage, CHP concentration is reduced to 0.5–5 weight percent and DMPC is converted to dicumyl peroxide (DCP) to the extent of at least 40 mol percent. The reaction is carried out in a stirred or back-mixed reactor. Preferred conditions are: temperature 50–90° C., water 0.4–4.5 weight percent, acid 50–75 ppm. In the second stage, CHP concentration is reduced to below 0.4 weight percent by passage of the reaction mixture through a plug-flow reactor essentially isothermal to the first stage. In the third stage, both DCP and the remaining DMPC and CHP are decomposed by heating the reaction mixture to 120–150° C. in a plug-flow reactor.
In both the aforementioned patents a key element is the presence of relatively large concentrations of residual CHP in the first stage reactor. In fact we have found that the ultimate yield of AMS from DMPC in the three stage process of U.S. Pat. No. 4,358,618 generally increases as the concentration of residual CHP in the first step is increased. Unfortunately, the higher the concentration of CHP in a stirred or back-mixed reactor, the less stable is the operation of the reactor, particularly in a large scale reactor. This is due to the fact that CHP decomposition is highly exothermic, and at the same time it is accelerated by increasing temperature. Consequently, when a relatively large concentration of residual CHP is present, the opportunity exists for a large release of thermal energy if the reaction is accelerated by a hot spot, a local surge of catalyst or other ill-controlled events. In typical commercial back-mixed reactors stable operation is very difficult to achieve at average residual CHP concentrations greater than 2–3 weight percent.
British Patent 1,202,687, to Societa' Italiana Resine S.P.A. teaches that formation of cumyl phenol and other undesirable condensates can be suppressed by carrying out CHP decomposition at 300 to 70° C. with acetone and an aqueous solution of sulfuric acid of concentration 10 to 75 weight percent resulting in a reaction product that contains 37 to 48 weight percent of acetone and 0.05 to 1.0 weight percent of sulfuric acid. The reaction is carried out in a single stage. We have found that under the broad conditions specified by said patent it is possible to obtain a reaction product that contains significant amounts of residual CHP. This is the case, for example, if the reaction is carried out at 30° C. with 10% aqueous sulfuric acid, and the resulting reaction product contains 0.5 weight percent sulfuric acid and 48 weight percent acetone. It will be appreciated, however, that mixtures containing highly reactive compounds such as CHP are not appropriate for subsequent isolation of reaction products by conventional procedures such as distillation. It is clearly the intent of said patent to effect substantially complete decomposition of CHP before product isolation. We have found that when all CHP is decomposed in a single stage within the operating conditions specified by said patent, significant amounts of cumyl phenol and AMS dimers are formed. Moreover, some DCP is also formed which in the absence of a second stage designed to decompose DCP, further decreases the yields of useful products. If operating conditions are modified towards the regime of lower reactivity (e.g. less acid, lower temperature) in an effort to suppress formation of cumyl phenol and AMS dimers, the rate of DCP formation increases. Thus the process taught in said patent cannot be used to increase the yield of useful product beyond a certain point.